In a communication system a communication network is provided, which can link together two communication terminals so that the terminals can send information to each other in a call or other communication event. Information may include audio, text, image or video data.
Modern communication systems are based on the transmission of digital signals. Analogue information such as speech is input into an analogue to digital converter at the transmitter of one terminal, hereinafter referred to as the near end terminal, and converted into a digital signal. The digital signal is then encoded and transmitted in data packets over a channel to the receiver of a destination terminal, hereinafter referred to as the far end terminal.
To transmit audio signals, such as speech, analogue audio data is input from a microphone at the near end terminal. The analogue audio data is then converted into digital data before it is transmitted to the far end terminal via the communication network.
A reply signal which is transmitted from the far end terminal, herein after referred to as the far end signal, is received at the near end terminal and output from a loudspeaker of the near end terminal.
A phenomenon commonly referred to as acoustic echo occurs when the far end signal output from the loudspeaker traverses an echo path and is recorded by the microphone of the near end terminal as an acoustic echo component in the near end signal. The echo component in the near end signal may in some cases cause the far end speaker to hear their own voice transmitted back from the near end terminal.
The echo path describes the effects of the acoustic paths travelled by the far end signal from the loudspeaker to the microphone. The far end signal may travel directly from the loudspeaker to the microphone, or it may be reflected from various surfaces in the environment of the near end terminal. The echo path may also describe any other effects that the far end signal has on the near end recording. For example the far end signal may cause mechanical vibration in the near end terminal, or cause electrical induction in the components of the near end terminal.
The echo path traversed by the far end signal output from the loudspeaker may be regarded as a system having a frequency and a phase response which may vary over time. By considering the echo component as the output of the system and the far end signal as the input of the system the frequency response of the echo path is a measure of the gain between the magnitudes of the output and the input of the system as a function of frequency.
FIG. 3 is a graph showing the frequency response of an echo path. The graph shows the gain in decibels (dB) of the output of the system as a function of frequency. The gain will typically be less than one because the echo path will attenuate the signal output from the loudspeaker.
In order to remove the acoustic echo from the signal recorded at the near end microphone it is necessary to estimate how the echo path changes the desired far-end loudspeaker output signal to an undesired echo component in the input signal. The effects of the echo path are estimated by calculating a mathematical representation of the relation between the signal output from the loudspeaker and the undesired echo input signal. The mathematical representation of the combined effects of the frequency and phase response which describes the echo path are hereinafter referred to as the echo path transfer function. When the echo path transfer function is accurately determined, the frequency response of the echo path transfer function will be equivalent to the frequency response of the actual echo path.
The echo path transfer function H(s) is the linear mapping of the Laplace transform X(s) of the far end signal to the Laplace transform Y(s) of the echo signal:
                                          Y            ⁡                          (              s              )                                =                                    H              ⁡                              (                s                )                                      ⁢                          X              ⁡                              (                s                )                                                    ⁢                                  ⁢                                  ⁢        or                            Equation        ⁢                                  ⁢                  (          1          )                                                  H          ⁡                      (            s            )                          =                                            Y              ⁡                              (                s                )                                                    X              ⁡                              (                s                )                                              =                                    L              ⁢                              {                                  y                  ⁡                                      (                    t                    )                                                  }                                                    L              ⁢                              {                                  x                  ⁡                                      (                    t                    )                                                  }                                                                        Equation        ⁢                                  ⁢                  (          2          )                    
The echo path transfer function H(s) is calculated by comparing the far end loudspeaker signal with the near end signal recorded by the microphone. When the near-end speaker is silent and the far-end speaker is active, only the echo provided by the far end signal is recorded by the near end microphone. In this case, the echo path transfer function can be adaptively calculated to model the way that the far-end signal changes when traversing the echo path.
In known acoustic echo cancellation (AEC) techniques the adaptively calculated echo transfer function is used to provide filter coefficients that filter the far end signal to generate an estimate of the echo component in the near end signal in accordance with the echo path transfer function. The estimated echo may then be subtracted from the near end signal. Other AEC techniques employ attenuation based filtering methods that attenuate the near end signal according to the calculated echo path transfer function to remove the echo component from the near end signal.
If the near-end signal is overloaded this causes a non linear distortion of the near end signal which substantially affects the accuracy of the calculated echo path transfer function by a tendency to underestimate the frequency response. As a result the residual echo in the near end signal transmitted to the far end terminal is likely to be high immediately after the overload. A signal overload occurs when the components of the terminal in the signal processing path are subjected to a greater load than they were designed to handle. A signal overload may be caused by a sudden increase in the signal power, a high gain setting, or by a slow reacting automatic analogue gain control. Signal overload is most likely to happen while the terminal transmitting the signal is adapting its settings during the initial stages of the communication.
Immediately after a signal overload occurs, the adaptive calculation of the echo path transfer function will take time to adapt to the non overloaded signal. During this time the echo component removed from the near end signal will be inaccurate and can cause an echo to appear at the far end terminal.
In some prior art methods, such as that disclosed in U.S. Pat. No. 6,850,783, the coefficients of the filters are prevented from adapting during periods of overload. This prevents the calculation of the echo path transfer function from altering during the overload period.
Whilst this method may prevent the echo estimation from significantly worsening during the overload, the presence of a high residual echo is not eliminated either during the overload or immediately afterward.
It is therefore an aim of the, present invention to overcome the problems presented by the prior art and to reduce the echo present in the transmitted signal during and immediately following signal overload.